1. Field of the Invention
The invention relates to endless belts for conveyors and, more particularly, to an endless conveyor belt using raked teeth or sheaves to be driven by a pulley.
2. Description of the Related Art
Conveyors with friction-driven flat belts are known systems for moving items from one location to another. A tensioned, endless belt extends between a drive pulley and a tail piece (typically a pulley or a fixed bar), whereby friction between the drive pulley and the belt enables transfer of torque from the former to the latter to thereby induce movement of the belt. Because tension on the belt is required to maintain the requisite friction for moving the belt, this type of conveyor does not perform well in environments where the tension and friction can be compromised. For example, the introduction of oil, grease, or other effluents from products carried on the belt can result in a loss of friction and thereby detrimentally affect the performance of the conveyor.
Another type of conveyor comprises a direct or positive drive modular belt. In this type of conveyor, a modular belt formed of a plurality of interlocking links extends between a drive pulley and an idler pulley and comprises a plurality of teeth that engage corresponding sheaves on the drive pulley, or alternatively, teeth on the drive pulley engage the links or sheaves on the belt. Interaction between the teeth and sheaves transfers torque to the belt. As a result, the conveyor does not rely on friction for moving the belt, and friction reducing compounds do not affect performance in the manner described above for friction-driven belts. However, low tension, direct drive modular belts are difficult to clean and to maintain. They are also porous and therefore cannot readily carry products such as powders and the like.
Conveyors with low friction, positive drive endless belts 100 having a flat surface 102 on one side and teeth 104 on the other side, as illustrated in FIG. 1, overcome the problems associated with the friction-driven flat belts and the modular belts. The seamless flat surface 102 is generally made of a thermoplastic material, non-porous and easy to clean, while the teeth 104 engage sheaves 106 on a drive pulley 108 to transfer torque to the belt 100 without requiring friction between the belt 100 and the drive pulley 108 or tension in the belt 100. Such a conveyor is disclosed in U.S. Patent Application No. 60/593,493, which is incorporated herein by reference in its entirety.
To account for belt stretching, it has been determined that the tooth pitch of the belt must be less than the sheave pitch of the drive pulley at less than maximum elongation of the belt. Also, the pulley pitch must equal the pitch of the belt at maximum elongation, give or take a fraction of a percent. Moreover, to ensure that the belt teeth are positioned to enter the pulley sheaves, the longitudinal width of each sheave in the pulley must exceed the belt tooth longitudinal width at least by the amount of distance generated by elongating the belt the maximum allowable amount over the span of the belt wrap. As a result of the pitch and width differences, the teeth and the sheaves will be longitudinally aligned as long as the elongation is at or below the maximum elongation.
Due to the pitch difference between the belt and the pulley, only one belt tooth will be driven by a pulley sheave at any given moment. It has been found that this engaged tooth is always the tooth that is about to exit the pulley. For all subsequent belt teeth that enter the pulley sheaves at any given moment, there is a gap between the driving face of the belt tooth and the driving face of the pulley sheave, and that gap progressively increases in size for each successive tooth. Consequently, as the exiting tooth disengages from the drive pulley, there remains some amount of gap between the following belt tooth, i.e., the trailing tooth, and the face of its respective pulley sheave. At this time, the pulley continues to rotate relative to the belt without moving the belt, and the effective drive characteristics are lost until the driving face of the sheave abuts the driving face of the trailing tooth. In other words, the pulley rotates while the belt slips until a tooth engages again. Discounting any momentum of the belt and any friction between the belt and the pulley, the belt will effectively stop for a brief moment until the following sheave engages the trailing tooth, which thereby becomes the new “exit tooth”.
Some slip between the belt and the pulley is what enables a direct drive application to work. This temporary disengagement of belt teeth from pulley sheaves causes the average belt speed to be less than the average pulley speed. In fact, the average belt speed is less than the pulley speed by the percentage of elongation that is still available in the belt (maximum elongation—current elongation). Because of this necessary slip, any friction between the pulley and the belt will compromise the benefits of direct drive. Friction between the belt and the pulley will retard slippage and can cause the trailing tooth to miss the pulley sheave altogether. To avoid such friction, the belt and the pulley can be made of, or coated with anti-friction materials, the pulley can be designed such that the belt and pulley have reduced contact area between the sheaves, and the belt is preferably maintained under low tension.
Also, to ensure that the engaged (driven) tooth stays engaged until the appropriate time to exit the sheave, a position limiter is used adjacent the belt.
Long belt runs in conveyor applications such as those moving coal, ore, or gravel typically require heavy, reinforced belts to minimize stretching and large drive motors to move the heavy belt as well as the load on the belt. The foregoing thermoplastic, low friction, direct driven belt has not been shown to be conducive to long belt runs. And the complexity of requiring position limiters to keep the drive tooth engaged with the pulley would only serve to complicate such a system in the harsh environment of moving coal, ore, or gravel.